It is well known that water can react with soda-lime-silica glass, very slowly, leaching sodium ions from the glass and forming sodium hydroxide, which raises the pH of the water in contact with the glass surface as follows: EQU SiONa (glass)+H.sub.2 O.fwdarw.SiOH (glass)+NaOH
If a small volume of water is left in contact with a glass surface for a prolonged period, as can occur in a stack of glass sheets in transit or storage, the pH can become highly alkaline. At pH levels above about 9.0, the hydroxide ions can cause corrosion of the glass surface by destroying silicon-oxygen bonds as follows: EQU SiOSi (glass)+OH.sup.- .fwdarw.SiOH+OSi.sup.-
literally dissolving the glass which results in a hazy or iridescent corroded glass surface. Since transportation and storage conditions cannot always be controlled, it has been common practice for glass manufacturers to use a variety of paper or powder interleaving materials between the surfaces of stacked glass sheets to retard stain damage. Such interleaving materials provide physical separation of the glass sheet surfaces, to minimize mechanical damage, such as abrasion, and also may comprise acid compounds to neutralize the hydroxide formed from alkali ions reacting with water, and retard the pH increase which leads to staining of the glass surface. A common interleaving material comprises polymethylmethacrylate beads for physical separation and adipic acid for neutralizing the hydroxide. Glass sheets may be stacked for transportation and/or storage in a variety of pack, box, pallet or rack configurations. A preferred shipping rack is described in U.S. Pat. No. 5,379,904, the disclosure of which is incorporated herein by reference.
U.S. Pat. No. 4,487,807 to Duffer et al. discloses protecting glass surfaces in a stack of glass sheets by treating the surfaces with a mixture of stain-inhibiting organic acids which crystallize on the glass surface, and separating adjacent glass sheets with an interleaving material, preferably in particulate form, such as synthetic polymeric beads or natural porous cellulose materials such as wood flour.
U.S. Pat. No. 4,489,106 to Duffer et al. discloses a two-step method for protecting glass surfaces in a stack of glass sheets by first treating the glass surfaces with a solution of stain-inhibiting organic hydroxy acid and drying the surface prior to dispersing a finely divided particulate interleaving material on the glass surface.
U.S. Pat. No. 4,529,648 to Duffer et al. discloses a method for applying a powdered interleaving material to a glass surface in the form of an aqueous slurry. The powdered interleaving material is preferably a porous cellulose material, such as wood flour or rice flour, which may also comprise a stain-inhibiting acid material such as boric acid, citric acid or tartaric acid.
U.S. Pat. Nos. 4,530,889 and 4,568,605 to Duffer et al. disclose methods and compositions to reduce the staining of stacked glass sheets utilizing porous particulate interleaving materials treated with strong organic acids, such as organotin halides and hydroxy carboxylic acids, respectively.
The above stain prevention techniques share significant disadvantages. The low solubility of some organic acids requires application of additional acid in powder form, and in cold weather it may be necessary to heat the solution to prevent precipitation of acid, clogging lines and nozzles. The very low pH of the acid solutions used to prevent stain by neutralizing alkali as it forms from sodium diffusion from the glass substrate causes corrosion of metal equipment used to apply the acid solution and process the glass.